Display technologies being developed by Dolby Laboratories, Inc., and others, are able to reproduce images having high dynamic range (HDR). Such displays can reproduce images that more faithfully represent real-world scenes than conventional displays.
To support backwards compatibility as well as new HDR display technologies, multiple layers may be used to deliver video data from an upstream device such as a multi-layer video encoder to downstream devices. Standard dynamic range (SDR) video data carried in a base layer (BL) of the multiple layers is optimized for viewing experience on SDR displays, while visual dynamic range (VDR) video data carried in the combination of the base layer and an enhancement layer (EL) of the multiple layers supports viewing experience of VDR displays having higher dynamic ranges than that of SDR displays. As used herein, codecs involved in encoding and decoding such image data are denoted as VDR codecs optimized for SDR displays.
BL image data may comprise lower bit depth (e.g., 8 bits per color component) SDR images derived from higher bit depth (e.g., 12 or more bits per color component) HDR source images from an image data input. The SDR images encoded in the BL image data typically comprise color-corrections by colorists to make the SDR images look as realistic as possible within a relatively narrow or standard dynamic range. For example, hue information related to some or all of the pixels in an input HDR image may be changed or corrected in an SDR image in order to create a realistic looking image within the standard dynamic range. These color corrections result in asymmetric clippings in various color channels, and introduce manual color alterations especially in relatively underexposed or overexposed regions of the HDR source images. The color corrected SDR image may allow SDR displays to show image details in the dark areas and highlights of an HDR source image.
Clipping is a type of color alternation that alters/modifies out-of-bound pixel values in color channels so that the resultant pixel values are within a target represented range (which may be one within a range supported by a specific type of SDR displays, or within a range supported by a range of SDR displays, or within a range supported by a range of VDR displays, etc.). Clipping may occur in zero, one or more of color channels (e.g., R, G, and B pixel values in a RGB color space in a certain portion of a HDR image may be clipped in a tone-mapped image). Amounts of clipping may or may not vary with the color channels (e.g., more clipping for green, less clipping for blue, etc.).
Color corrections, such as clipping, introduced into SDR images make the SDR images to comprise different and independently sourced image contents from their counterpart VDR images, and are difficult and even impossible to remove by a downstream device for the purpose of reconstructing high dynamic range images without complicated processing and without a sufficiently large bitrate. When multiple layers are used to transmit image data to a downstream device, reversing color corrections may require a large volume of additional image data to be transmitted, for example, in an enhancement layer, to the downstream device.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.